Abstract
Among coupled exchangers, CLCs uniquely catalyze the exchange of oppositely charged ions (Cl- for H+). Transport-cycle models to describe and explain this unusual mechanism have been proposed based on known CLC structures. While the proposed models harmonize with many experimental findings, gaps and inconsistencies in our understanding have remained. One limitation has been that global conformational change - which occurs in all conventional transporter mechanisms - has not been observed in any high-resolution structure. Here, we describe the 2.6 Å structure of a CLC mutant designed to mimic the fully H+-loaded transporter. This structure reveals a global conformational change to improve accessibility for the Cl- substrate from the extracellular side and new conformations for two key glutamate residues. Together with DEER measurements, MD simulations, and functional studies, this new structure provides evidence for a unified model of H+/Cl- transport that reconciles existing data on all CLC-type proteins.
Highlights
CLC transporter proteins are present in intracellular compartments throughout our bodies – in our hearts, brains, kidneys, livers, muscles, and guts – where they catalyze coupled exchange of chloride (Cl–) for protons (H+) (Jentsch and Pusch, 2018)
In contrast to the single-point mutants of Gluin and Gluex, which reveal either no conformational change (Glnin) (Accardi et al, 2005) or only a simple side-chain rotation (Glnex) (Dutzler et al, 2003), the QQQ mutant structure reveals global conformational change, which generates the expected opening of the extracellular permeation pathway. This structure reveals new side-chain conformations for both Glnex and Glnin. Based on this new structure, together with MD simulations, DEER spectroscopy, and functional studies, we propose an updated framework for modeling the CLC transport cycle
It was suggested that this ‘out’ position may be relevant only to CLC channels, due to the steric clashes with conserved residues that the ‘out’ conformation would generate based on known CLC transporter structures (Park and MacKinnon, 2018)
Summary
CLC transporter proteins are present in intracellular compartments throughout our bodies – in our hearts, brains, kidneys, livers, muscles, and guts – where they catalyze coupled exchange of chloride (Cl–) for protons (H+) (Jentsch and Pusch, 2018) Their physiological importance is underscored by phenotypes observed in knockout animals, including severe neurodegeneration and osteopetrosis (Sobacchi et al, 2007; Stobrawa et al, 2001; Hoopes et al, 2005; Kasper et al, 2005), and by their links to human disease, including X-linked mental retardation, epileptic seizures, Dent’s disease, and osteopetrosis (Lloyd et al, 1996; Hoopes et al, 2005; Veeramah et al, 2013; Hu et al, 2016). Like all CLC proteins, CLC-ec is a homodimer in which each subunit contains an independent anion-permeation pathway
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